1. Field of the Invention
This invention relates generally to a catalytic combustor for a gas turbine engine, and in particular, to a fuel-air mixer for a catalytic combustor for a gas turbine engine.
2. Description of Related Art
One widely used device for the generation of electricity, power, and heat is the gas turbine engine. A typical gas turbine engine operates by intaking air and pressurizing it using a rotating compressor. The pressurized air is passed through a chamber, or “combustor,” wherein fuel is mixed with the air and burned. The high temperature combustion of the fuel-air mixture expands across a rotating turbine, resulting in a torque created by the turbine. The turbine may then be coupled to an external load to harness the mechanical energy. Gas turbine engines are commonly used for electrical generators, and to power turbo-prop aircraft, pumps, compressors, and other devices that may benefit from rotational shaft power.
In a typical gas turbine engine, the combustion chamber, fuel delivery system, and control system are designed to ensure that the correct proportions of fuel and air are injected and mixed within one or more “combustors.” A combustor is typically a metal container, or compartment, where the fuel and air are mixed and burned. Within each combustor, there is typically a set of localized zones where the peak combustion temperatures are achieved. These peak temperatures commonly reach temperatures in the range of 3300 degrees Fahrenheit. These high temperatures also become the source of nitrogen oxide and nitrogen dioxide (NOx) emissions, a known pollutant. Typically, to prevent thermal distress or damage to these metallic combustion chambers, a significant amount of the compressor air passes around the outside of the combustors to cool the combustors. The air, which then drives the turbine, is a combined mix of the hot combustion gasses and this cooling air. The resulting hot gas yield, which is admitted to the inlet of the turbine, is delivered at a temperature in the range of 2400° F. at full load for a typical industrial gas turbine. Unfortunately, virtually all of the NOx produced in the peak temperature zones within the combustor is exhausted into the atmosphere.
One method for reducing NOx formation in the combustion processes of a gas turbine engine includes premixing the fuel and air. As the fuel-to-air ratio changes within a combustor, the NOx, formation within the combustor changes due to variations in the peak flame temperature and the availability of oxygen as the fuel-to-air ratio is altered. Premixing the fuel and air increases the uniformity of the fuel-air mixture and thereby provides temperature uniformity. The temperature uniformity minimizes the formation of high flame temperature zones and reduces the production of NOx.
Numerous mixers and mixing devices are known for premixing fuel and air in conjunction with conventional combustors. One type of mixer, which is often referred to as an “open mixer,” includes gas issuing from an orifice and being entrained with air within a long downstream region due to the kinetic energy of the high velocity flow path of the air and gas. Open mixers do not employ any internal obstructions and generally require the long downstream region for complete mixing. Another type of mixer includes internal baffles or swirlers, which divert flow paths to create shear and enhance turbulent mixing of the fuel and air without the long downstream region.
Drawbacks of conventional premixing designs, which employ internal baffles or swirlers, include flameholding, central vortex breakdowns, and recirculation in regions downstream of the baffles or swirlers. Flameholding generally refers to the premature auto-ignition of the fuel and air within the premixing region that is typically caused by insufficient flow velocity in the premixing region. Central vortex breakdowns and recirculation generally occurs in regions downstream from the baffles or swirlers, referred to as a diffuser region, and may be caused by too much or too little turbulence caused by the baffles or swirlers.